Datasheet LT1580, LT1580-2.5 (Analog Devices) - 8

LT1580/LT1580-2.5
U U W U APPLICATIONS INFORMATION
pin and the top of the resistor divider are connected to the values of RP. Trace B of Figure 3 illustrates the effect on top of the load. The bottom of the resistor divider is output regulation. It is important to note that the voltage connected to the bottom of the load. R drops due to R P is now effectively P are not eliminated. They will add to the connected inside the regulating loop of the LT1580 and the dropout voltage of the regulator regardless of whether load regulation at the load will be negligible for reasonable they are inside the loop as in Figure 2 or outside the loop as in Figure 1. This means that the LT1580 can control the 5V voltage at the load as long as the input-output voltage is greater than the total of the dropout voltage of the LT1580 VCONTROL plus the voltage drop across R 3.3V V SENSE POWER P. LT1580
Stability
V + OUT The LT1580 requires the use of an output capacitor as part R ADJ P of the device frequency compensation. The device re- LOAD VOUT quires a minimum of 22 R1 µF tantalum or 150µF of aluminum R2 R electrolytic to ensure stability. Larger capacitor values P – increase stability and improve transient performance. 1580 F01 Many different types of capacitors are available and have
Figure 1. Conventional Load Sensing
widely varying characteristics. These capacitors differ in capacitor tolerance (sometimes up to 5V ±100%), equivalent series resistance, equivalent series inductance and ca- pacitance temperature coefficient. The LT1580 frequency VCONTROL 3.3V V SENSE POWER compensation optimizes frequency response with low ESR capacitors. In general, use capacitors with an ESR of LT1580 less than 1Ω. V + OUT R ADJ P For microprocessor applications larger value capacitors will be needed to meet the transient requirements of the LOAD VOUT R1 processor. Processor manufacturers require tight voltage R2 RP tolerances on the power supply. High quality bypass – 1580 F02 capacitors must be used to limit the high frequency noise generated by the processor. Multiple small ceramic ca-
Figure 2. Remote Load Sensing
pacitors in addition to high quality bulk tantalum capaci- tors are typically required to limit parasitic inductance (∆IOUT)(RP) (ESL) and resistance (ESR) in the capacitors to acceptable V levels. The LT1580 is stable with the type of capacitors OUT FIGURE 1 recommended by processor manufacturers. VOUT Bypassing the adjust terminal on the LT1580 improves FIGURE 2 ripple rejection and transient response. The ADJ pin is IOUT brought out on the fixed voltage device specifically to allow this capability. TIME 1580 F03 Capacitor values on the order of several hundred microfar-
Figure 3. Remote Sensing Improves Load Regulation
ads are used to ensure good transient response with heavy 8